Hi!
I am very sorry that you have lost your boy. It could have well been a bad reaction to the GA (general anaesthesia), especially with vets that are not necessarily well practised in operating on guinea pigs and other small animals. The longer a piggy is under the greater the risk of complications from GA. But we can only speculate...
Your experience is not quite unique; we have seen quite a few sudden mystery deaths following an operation - although they have thankfully become rarer over the years.
The experience of your vet is the biggest factor in the operation success rate.
Guinea Lynx :: Finding a Veterinarian
Veterinarians: North Carolina - Guinea Lynx Records (recommended vets for your state in case you would rather look around)
I've currently got three spayed sows (for ovarian cyst issues) but have had sows spayed for cysts before that and a couple of sows have been spayed before I adopted them.
Thankfully, I do have access to a very good vet with great experience in this operation, so the ops have all come off and my sows have bounced back well straight away. Piggies operated as emergencies at my local general vet have had a bit of a rougher recovery due to a different. less expensive GA drug being used and my local vet being less practised with small animals.
In the case of spaying, there are several operation methods as to whether you are opting for a full spay including removal of the womb from the belly or having just the ovaries removed via small incisions from the back.
Personally, I don't mind which way; more important for me is that my operating vet is feeling happy about their operation method.
Any operation after an unsuccessful one gives you the jitters; that is perfectly normal. It usually takes a very deep breath. Even more so with elective surgery.